Tropical seagrasses are nutrient-limited owing to the strong phosphorus fixation capacity of carbonate-rich sediments, yet they form densely vegetated, multispecies meadows in oligotrophic tropical waters. Using a novel combination of high-resolution, two-dimensional chemical imaging of O₂, pH, iron, sulfide, calcium, and phosphorus, we found that tropical seagrasses are able to mobilize the essential nutrients iron and phosphorus in their rhizosphere via multiple biogeochemical pathways. We show that tropical seagrasses mobilize phosphorus and iron within their rhizosphere via plant-induced local acidification, leading to dissolution of carbonates and release of phosphate, and via local stimulation of microbial sulfide production, causing reduction of insoluble Fe(III) oxyhydroxides to dissolved Fe(II) with concomitant phosphate release into the rhizosphere porewater. These nutrient mobilization mechanisms have a direct link to seagrass-derived radial O₂ loss and secretion of dissolved organic carbon from the below-ground tissue into the rhizosphere. Our demonstration of seagrass-derived rhizospheric phosphorus and iron mobilization explains why seagrasses are widely distributed in oligotrophic tropical waters.

Visualization and quantification of analytes such as O₂ or pH is essential in biological research. Here we present the design and application of a new optical dual-analyte sensor for imaging, optimized to have low cross-sensitivity between the two analytes O₂ and pH. The used indicator and reference dyes were selected to match the different channels of a commercial 2CCD (RGB + NIR) camera. A red-light emitting O₂-sensitive europium complex (Eu(HPhN)₃dpp) with a dynamic range of 0–20% O₂ in the finished sensor was combined with a near-infrared emitting pH indicator (OHButoxy-aza-BODIPY) with a dynamic range of pH 7.2–8.8. To enable ratiometric readout, an inert reference coumarin dye (Bu₃Coum) was co-immobilized with the optical indicators. In order to maximize the sensor signal, inert diamond powder was added to one sensor layer as a simple way to increase scattering of light within the sensor. Furthermore, the addition of an optical isolation layer enabled measurements in highly fluorescent samples, such as algal biofilms. The sensor was tested in a marine photosynthetic microbial mat.

Far-red absorbing chlorophylls are constitutively present as chlorophyll (Chl) d in the cyanobacterium Acaryochloris marina, or dynamically expressed by synthesis of Chl f, red-shifted phycobiliproteins and minor amounts of Chl d via far-red light photoacclimation in a range of cyanobacteria, which enables them to use near-infrared-radiation (NIR) for oxygenic photosynthesis. While the biochemistry and molecular physiology of Chl f-containing cyanobacteria has been unraveled in culture studies, their ecological significance remains unexplored and no data on their in situ activity exist. With a novel combination of hyperspectral imaging, confocal laser scanning microscopy, and nanoparticle-based O₂ imaging, we demonstrate substantial NIR-driven oxygenic photosynthesis by endolithic, Chl f-containing cyanobacteria within natural beachrock biofilms that are widespread on (sub)tropical coastlines. This indicates an important role of NIR-driven oxygenic photosynthesis in primary production of endolithic and other shaded habitats.

Hydrogen peroxide (H₂O₂) plays a key role in many biological processes spanning from coral bleaching, over cell signaling to aging. However, exact quantitative assessments of concentrations and dynamics of H₂O₂ remain challenging due to methodological limitations—especially at very low (sub μM) concentrations. Most published optical detection schemes for H₂O₂ suffer from irreversibility, cross sensitivity to other analytes such as other reactive oxygen species (ROS) or pH, instability, temperature dependency or limitation to a specific medium. We review optical detection schemes for H₂O₂, compare their specific advantages and disadvantages, and discuss current challenges and new approaches for quantitative optical H₂O₂ detection, with a special focus on luminescence-based measurements. We also review published concentration ranges for H₂O₂ in natural habitats, and physiological concentrations in different biological samples to provide guidelines for future experiments and sensor development in biomedical and environmental science.